Internal combustion engines that use a single intake valve and a single exhaust valve typically are not capable of producing the same intake port area as an internal combustion engine having a pair of intake valves. Thus, intake air/fuel mixture flow for a two-valve engine is less than the corresponding air/fuel mixture flow for a four-valve engine, even in those instances when the two-valve engine valve size has been increased to the maximum practical diameter. Further, the rate of valve opening for a two-valve engine, to achieve a flow comparable to the flow of a four-valve engine of equal displacement, is inherently greater than the rate of valve opening for a four-valve engine. Substantially more valve lift, therefore, is required to achieve the same flow compared to a four-valve engine.
One of the advantages of using a two-valve engine is the inherent ability of a two-valve combustion chamber, with offset intake port locations, to generate an air/fuel mixture swirl motion. This feature contributes to a fast and effective burn at low and moderate engine speeds. Because of this characteristic, better performance at the low speed end of the engine torque output curve can be achieved compared to a four-valve design.
In the case of a four-valve engine, the breathing characteristics at the top power output end of the engine speed curve is superior to the breathing characteristics of a two-valve engine, but low and mid-load range operating characteristics may be less than favorable compared to the corresponding characteristics of a two-valve engine.
One of the characteristics of a four-valve engine that causes inferior performance at the low speed of the engine power range is the difficulty in obtaining an effective swirl motion for the air/fuel mixture to achieve optimum mixing for efficient and rapid combustion. Although a tumble motion of the air/fuel mixture can be achieved in the case of a four-valve design, the effectiveness of the burning of the tumbling air/fuel mixture is less than the comparable effectiveness of a swirl motion of the air/fuel mixture. Even if a swirl motion can be achieved in a four-valve design (e.g., by using an auxiliary charge motion control valve to disable one intake valve port), a central location of the spark plug allows the swirl pattern to encircle the ignition source, which detracts from the effectiveness of the burn notwithstanding the presence of swirl.